The present invention relates to devices used with a particle beam for measuring beam current with a Faraday cup in general, and in particular to systems which determine the beam currents of two or more different particle beams using the same Faraday cup.
A beam of accelerated particles is useful in many scientific and industrial applications, from ion implantation for controlling material properties, to investigating fundamental principles of physics, to accurately dating ancient materials or following chemical reactions or fluid flows by monitoring isotope ratios. A mass spectrometer is one widely used application of particle beams. A mass spectrometer can be used to detect the ratio between the isotopes of a particular element. When one of the isotopes is radioactive, it typically has a low abundance in the environment because it must be constantly supplied from a source, such as the atmosphere or a parent radioactive material, or it disappears over time. Thus it is possible to date a material from the time the material was removed from a source of a radioactive isotope e.g., carbon-14 in the atmosphere. Various chemical and biological processes can cause a separation of isotopes, so that measurement of non-radioactive isotope ratios can be used to determine the diet of ancient animals from their remains, as well as the temperature or other conditions under which they lived.
One of the most widely used isotope determinations is for 14C. Carbon-14 is constantly generated in the upper atmosphere through the interaction of neutrons produced by cosmic rays with Nitrogen 14. 14C forms carbon dioxide which taken up by plant life and so is incorporated into all living things. The amount of radioactive 14C in the atmosphere has remained relatively stable over a period of thousands of years. However, once living matter dies, it no longer exchanges carbon with the atmosphere and so the amount of 14C gradually decreases in accord with the half-life of 14C of about 5,730 years. By determining the ratio of 14C to non-radioactive 12C and 13C in an ancient sample, and comparing with the same ratio in a modem carbon sample from living material, it is possible to determine how many years have transpired since the source of the carbon in the ancient sample died.
Since the late 1970s tandem electrostatic accelerators have been used as extremely sensitive mass spectrometers able to distinguish the atomic isotopic ratios of 17 orders of magnitude or more. For example, in a modem sample of carbon the ratio is 1.35×10−12 between carbon-12 and carbon-14. Radioactive isotopes with long half-lives are difficult to measure by detection of radioactive decay if the sample size is small and the half-life of the radioactive isotope is large. For radioactive carbon-14, with a half-life of 5,730 years, a sample size of about one gram is generally considered necessary for conventional radioactive carbon dating. A one-gram sample of modem carbon contains approximately 10−12 grams 14C or approximately 5×1010 atoms of 14C and produces only 14 disintegrations per minute. By using an accelerator mass spectrometer (AMS), as much as 10 percent of the atoms of 14C present in a sample can be directly detected. The result is that the concentration of 14C can be measured with a precision of better than one percent in a modem sample, using a sample size of less than one milligram in only a few minutes. The ability to uniquely detect the presence of atomic isotopes finds many uses beyond carbon dating, for example using atomic isotopes as chemical labels.
Mass spectrometry uses the principal that a charged particle is deflected more or less by a magnetic or static electric field depending on the velocity and mass of the particle. By the proper combination of magnetic and/or electrostatic analyzers it is possible to separate particles by mass and charge and thus to detect the mass and energy of individual particles. The unique detection of a particular atomic isotope, however, requires that all molecular isobars be eliminated. For example, in the case of 14C molecular isobars of 13CH and 12CH2 are perhaps one million times more prevalent than the 14C to be measured. To detect 14C, negatively charged particles of mass 14 are accelerated in the tandem accelerator through a potential of about one-half million volts to several million volts. The negatively charged particles of mass 14 are passed through a stripping column of rarefied gas in the high voltage positively charged electrode. The stripping column causes the particles to lose electrons and in the process breaks up any molecular isobars into their constituent parts. The positively charged ions are accelerated away from the positively charged high voltage electrode to ground and the particles of mass 14 are separated and counted.
The isotopes of carbon occurring in nature are approximately 99 percent 12C, 1 percent 13C, and 10−13 percent 14C. To obtain accurate determinations the amount of 14C present it is necessary to compare the amount of 14C detected to the amount of 13C and 12C present in the sample because it is the ratio, not the absolute amount of 14C which is of interest. The amount of each carbon isotope measured while substantial is only about 10 percent of that which was originally contained in the sample. To obtain accurate results it is necessary to precisely measure the relative abundance of all three isotopes to account for all the loss mechanisms, which can effect the different isotopes of Carbon differently. In order to minimize the amount of power consumed by the high voltage electrode and minimize generation of radiation, it is desirable to minimize the beam currents which are accelerated. This is typically accomplished by accelerating the 12C, and 13C only for short periods of time between longer periods of time during which the 14C beam is injected and analyzed.
To minimize the overall size of the beam transport system on the injection side of the accelerator, a single Faraday cup which receives both the 12C and 13C beams can be utilized. It has also been known to use two parallel analog circuits to measure the beam current supplied to the common Faraday cup. Compensation for the differences in beam current between the 13C and the 12C is done by reducing the relative length of the 12C beam pulse with respect to the 13C beam pulse. Prior art analog integration circuits used several operational amplifiers which added to offset and gain errors, as well as limited gain adjustments. What is needed is a more accurate and flexible circuit for measuring beam currents over a wide range of beam currents.